define(function () {
  // 定义math模块中的sum方法
  var sum = function (x, y) {
    return x + y;
  }
  // 定义math模块中的diff方法
  var diff = function (x, y) {
    return x - y;
  }
  // 把定义的math方法返回出去
  return {
    sum: sum,
    diff: diff
  }
});
